Apparatus and method for selectively producing high current of high energy beams of accelerated charged particles



J. HAIMSON APPARATUS AND METHOD FOR SELECTIVELY PRODUCING HIGH CURRENTOF HIGH ENERGY BEAMS OF ACCELEHATED CHARGED PARTICLES 2 Sheets-Sheet 1Filed Sept. 28, 1964 T .z: EEEw $5.1 3

INVENTOR JACOB HAIMSON APPARATUS Filed Sept. 28, 1964 J. HAIMSON3,398,375

2 Sheets-Sheet 2 s4 85 as 7 l a s1 H 2 I F IGBA CATHODE PULSE 65 w FIG.3 I E W .n i n 63 s7 67 I EE fit INJECTED H -s| g PULSE 52 48 7//// M685 1- m INVENTOR rjfiqnm ON 6 BY m ORNEY United States Patent 3,398,375APPARATUS AND METHOD FOR SELECTIVE- LY PRODUCING HIGH CURRENT OF HIGHENERGY BEAMS 0F ACCELERATED CHARGED PARTICLES Jacob Haimson, East PaloAlto, Calif., assignor to Varran Associates, Palo Alto, Calif., acorporation of California Filed Sept. 28, 1964, Ser. No. 399,930 8Claims. (Cl. 328233) ABSTRACT OF THE DISCLOSURE In a linear particleaccelerator having at least three accelerating sections, a dual outputklystron provides input power into the first section and into the secondor third sections depending on whether a high current or high energyoutput is desired. In the high current mode, the input power is coupledinto the second section through a phase shifting switching circuit withthe residual energy remaining at the output of the first section beingdissipated in an external load. In the high energy mode, the phaseshifting switching circuit couples the input power into the thirdsection with the residual energy remaining at the output of the firstsection being coupled through a second phase shifting switching circuitinto the second section instead of the load. A heater is provided in thecooling lines of the third section to control the temperature andelectrically decouple the third section during high current operation inrder to effect a zero net energy transfer from the particle beam to thewaveguide section as the beam passes therethrough.

The present invention relates in general to particle accelerators andmore particularly to multiple section linear particle accelerators.

In the acceleration of particles such as, for example, electrons andpositrons to high energy in a linear particle accelerator, the ultimatebeam energy and beam current are dependent upon the impedance and lengthof the accelerating waveguide and the applied RF power. Alternately, thelength and impedance characteristics of the waveguide structures aredependent upon the required beam energy and the efiiciency of conversionfrom RF power to particle beam power. Also because the input RF power islimited from each klystron RF power source to, on the order of -30 mw.,high power beam performance requires multiple klystrons and acceleratingwaveguide sections. It is often desired to utilize the particle beamover a wide variation of current and energy levels at the end of theseveral accelerator sections.

The object of the present invention is to provide a multiple sectionlinear accelerator that will produce multiple performance specificationswithout changing the parameters of the injection and bunching systems ofthe accelerator.

One aspect of the present invention is the provision of a dual purposeaccelerating structure which can be adjusted for multiple performancespecifications such as either high current, medium energy output or highenergy medium current output. In accordance with this aspect of thepresent invention a series of at least three accelerating sections areprovided with means for directing input power into the first section andother input power either into the second section for high currentoperation or into the third section for high energy output in which casemeans are provided for coupling the energy remaining at the end of thefirst section into the second section. For high current operation of theaccelerating structure provision is made for either displacing ordecoupling the third section for utilization of the 3,398,375 PatentedAug. 20, 1968 ICE particle beam current and energy emanating from thesecond section. This accelerator construction permits the trapping andbunching electric fields which vary from cavity to cavity in the initialaccelerating waveguide, to be utilized and maintained for both highcurrent and high energy operation.

Another feature of the present invention is provision for decoupling onesection of a plurality of accelerating waveguide sections from theparticle beam transmitted therethrough by changing the operatingtemperature of the section thereby to change the dimensions and thus thephase of the traveling RF wave with respect to the beam particle bunchesin such a manner that there is a zero net energy transfer from theparticle beam to the waveguide section upon passage therethrough.

These and other features and advantages of the present invention will bemore apparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic view partially broken away of a linear acceleratorincorporating features of the present invention;

FIG. 2 is an enlarged elevational sectional view of a portion of thestructure schematically illustrated in FIG. 1;

FIG. 3 is a sectional view of a portion of the structure shown in FIG. 2taken along line 33 in the direction of the arrows;

FIG. 4 is an enlarged sectional view of a portion of a structure shownin FIG. 2 taken along line 4-4 in the direction of the arrows;

FIG. 5 is an enlarged perspective view partially in sectionschematically illustrating a main chopping cavity in accordance with thepresent invention;

FIG. 6 is a sectional view of an alternative structure in accordancewith the present invention;

FIG. 7 is a sectional view of a portion of the structure shown in FIG. 6taken along line 77 in the direction of the arrows; and

FIGS. 8A-8C are graphs of current versus time for particles in certainportions of a machine employing the present invention.

The present invention is directed primarily to a pulsin-g apparatuswhich can be utilized in a particle accelera tor such as, for example,an electron linear accelerator. \Vhile the invention will be describedhereinafter with particular reference to an electron linear acceleratorit can be utilized with other pulse particle utilizing apparatus suchas, for example, positron accelerating structures.

Referring now to the drawings, with particular reference to FIG. 1,there is schematically illustrated a particle accelerator 10 whichincludes an elongate vacuum envelope 11 with a beam generating assembly12 for generating and directing a particle beam 13 longitudinally of theenvelope 11. A beam deflecting and injecting assembly 14 is disposedalong the envelope 11 between the beam generating assembly 12 and aparticle accelerating wave guide 15 which is provided at its output endwith an output window 16 through which a pulse of electrons which havebeen accelerated to relativistic velocities can be passed for performingsophisticated nuclear experiments or for direction onto a targetelectrode for generating other radiation such as, for example, X-rays.

The accelerating wave guide 15 includes a plurality of wave beaminteraction structure 17, 18 and 19 such as, for example, apertureddisc-loaded wave guide for transferring energy from radio frequencyelectromagnetic waves to charged particles such as, for example,electrons passing therethrough for accelerating the electrons torelativistic velocities. The accelerating wave guide 15 is energized bya high power RF source such as, for example, a dual output klystron tube21 in which into section 17 for transmission therethrough, through aphase shifter 22 and through section 18 for interaction with a pulse ofcharged particles directed therethrough from the beam generatingassembly 12. The accelerating sections 17, 18 and 19 are fluid-cooledsuch as, for example, by water for absorption of the heat dissipated inthe waveguide by the RF power, and residual RF power remaining at theend of section 18 can be coupled to a load 23. RF energy from the otheroutput arm of the klystron 21 is coupled through a phase shifter 24 intothe accelerating section 19 in proper phase relation with respect to thepartially accelerated pulse of charged particles to continueaccelerating the particles until they reach the end of the acceleratingsection 19 and are passed through the window 16 or into other apparatusfor utilization. Residual RF power at the output end of acceleratingsection 19 can be passed into an external load 25. The fluid-coolingassembly for the third accelerating section 19 is provided with a heater20 for changing the temperature of the cooling fluid, thereby todecouple the section 19 from the particle beam.

The beam generating assembly 12 and the beam deflecting and injectionassembly 14 are constructed for producing pulses of charged particleshaving a pulse length continuously variable down to a minute fraction ofa second.

Referring now to FIG. 2 which illustrates an enlarged view of assemblies12 and 13, the beam generating assembly 12 includes a cathode and focuselectrode assembly 31 which is mounted at the one end of the envelope 11and whichd uring operation is positioned in an electrical insulating oilbath 32. Spaced along the axis of the particle accelerator from thecathode of focus electrode assembly 31 is an apertured anode 33 which isfollowed by a beam chopping cavity resonator 34 which will be describedin greater detail below with reference to FIG. 5.

A drift space 35 is positioned along the beam path following the cavityresonator 34 and is surrounded by a magnetic lens 36 for focussing theparticle beam 13 onto a collector 37 or through a collimating aperture38 in the collector 37 for passing short pulses of charged particlesthrough the collimating aperture 38 into the beam deflecting andinjection assembly 14. The collector 37 and the anode 33 are providedwith water-cooling channels 39 and 41 respectively for passing coolingfluid through these respective electrodes to cool the electrodes whichare heated due to interception of the charged particles.

A plurality of separate beam steering probes are provided around theaperture in the anode 33 and include a steering rod 42 of magneticmaterial which projects radially outwardly of the anode 33 with itsexterior end surrounded by a coil 43 for changing the magnetic fieldestablished at the interior ends of the steering rods 42.

A coupling loop 44 is provided for coupling RF energy into the cavityresonator 34 which can be tuned to the desired operating electromagneticmode by means of the tuner schematically illustrated at 45.

The beam deflection assembly 14 spaced axially down the envelope 11 fromthe collector 37 includes a focusing lens 46 such as, for example, athin magnetic lens coil for focusing the diverging pulsed electron beam13 onto a subsequently positioned collecting electrode 47 or through anaxially aligned collimating aperture 48 in the collector 47. A firstpair of deflection plates 51 and 52 is located on opposite sides of thebeam path between the collector 37 and the lens 46 for deflecting thepulsed electron beam 13 from a position in which the beam impinges onthe collector 47 to a position for passage through the collimatingaperture 48. The collector 47 is provided with an energy distributingslot 49 (see FIG. 4), V-shaped in cross-section, extending across thecollector on opposite sides of the collimating aperture 48 forcollecting the beam focused thereinto over a large surface area. Thepulse of charged particles is swept longitudinally of the slot 49 duringoperation of the deflecting assembly as described in detail below. Thecollector 37 is provided with a similar energy distributing slot alongwhich the pulse of charged particles is swept by the fields in the beamchopping cavity resonator 34.

Each of the deflection plates 51 and 52 is semi-cylindrical in formtapering outwardly in the direction of beam travel and is supported onone end of a conducting rod 53, the other end of which is supported in avacuum seal assembly 54 located in the envelope 11. The conducting rod53 which supports the deflection plate 51 is connected to a thyratron 55which is coaxially supported with respect to the vacuum seal 54. Theconducting rod 53 which supports the deflection plate 52 is connected toground via a metallic cup-shaped member 56 which surrounds the vacuumseal 54' and is electrically connected to the body of the metallicenvelope 11.

A second pair of deflection plates 61 and 62 is located between the lens46 and the collector 47 for deflecting the electron beam from theposition in which it passes through the collimating aperture 48 to aposition for ampringement on the collector 47. These plates 61 and 62are shaped similarly to plates 51 and 52 but tapered inwardly toward theaxis in the direction from the lens 46 to the collector 47. Plate 61 isconnected via connecting support rod 63, through a vacuum seal 64 to acoxially mounted thyratron 65 while plate 62 is connected via supportrod 63', through vacuum seal 64' and via a metallic cup-shaped member 66to the envelope 11 and ground. The second pair of deflection plates 61and 62 is rotated (not shown) about the envelope 11 axis with respect tothe pair of plates 51 and 52 to allow for rotation imparted to the beamin passing through the lens 46, and this rotation is accomplished by arotatable vacuum joint in the envelope at the lens 46. This vacuum jointincludes a metal gasket vacuum seal 57 which is held together by a pairof rotatable flanges 58 rotatably secured on the envelope 11 on oppositesides of the magnetic lens 46 by retaining rings 59 and held together bya plurality of bolts 60.

A magnetic bias field is produced in the region between the second pairof plates 61 and 62 by a pair of electric coils 67 provided with endpole pieces '68 located against the exterior of the envelope 11 midwaybetween the deflection plates 61 and 62 as shown in greater detail inFIG. 3. The magnetic bias field produced by the coils 68 counterbalancesthe electric field between plates 61 and 62 so that for operation of thedeflection assembly as described in greater detail below plate 62 can begrounded, thereby avoiding problems of voltage variation on plate 62 andconsequent variation in electric field strength between the plates dueto interception of charged particles on plate 62.

The electric field between each pair of deflection plates can beseparately controlled as will be described in greater detail below fordeflecting the electron beam between the position off the axis of theenvelope 11 for collection on collector 47 to a position on the axis ofthe envelope 11 for passage through the collimating aperture 48. In thismanner an easily adjusted short pulse of electrons can be passed throughthe collimator 48 into a prebuncher cavity 71 in which RF fields areestablished such as, for example, by an RF signal introduced thereintovia an input coupling loop 72. The bunched short pulse of chargedparticles emanating from the prebunching cavity 71 is focused to a smalldiameter by means of, for example, a thin magnetic lens coil 73 anddirected into the input end of the first accelerator section 17 througha collimating aperture 74. The focusing coil 73 is axially slidablealong the length of a drift space 75 between the prebunching cavity 71and the collimating aperture 74 for producing an optimum focus of thepulse of charged particles into the accelerating structure.

Referring now to FIG. 5, the chopping cavity 34 has oscillating magneticfields across the beam path through the center of the cavity and thisregion of the cavity is free from counterbalancing deflecting electricfields. As shown, the cavity resonator 34 is a rectangular cavityoperating in the TE mode. The electric fields associated with thedeflecting magnetic fields illustrated in FIG. 5 are threaded throughthe magnetic fields and rather than existing at the center of the cavitywhere the beam path lies are distributed such as to provide a peak fieldbetween the beam hole and the cavity end walls. Alternatively, thecavity resonator can be circular resonator operating in the TM mode forproducing the same particle deflection eflects as the rectangular cavitydescribed above. Here again in the circular TM cavity resonator theoscillating magnetic fields arranged transverse to the cavity axis inthe pattern as shown in FIG. 5 for a rectangular cavity are concentratedon the beam axis of the accelerator with the electric fieldsconcentrated remote therefrom. With the particle beam directed throughthe cavity resonator centrally thereof, the particle beam is subjectedto deflecting magnetic fields without being subjected to compensating orcounterbalancing deflecting electric fields.

Chopping cavities operating in higher order modes such as, for example,rectangular TE and circular TM where n and m are integers and n22 can beutilized so long as the relationship of beam path to the magnetic andelectric fields is similar as for the primary modes described above.

Operation of the method and apparatus in accordance with the presentinvention will be described with a typical operating example. A pulsedelectron beam is produced in the beam generating assembly 12 having apulse duration of approximately several microseconds, a rise time on theorder of tenths of microseconds, and a peak beam current on the order ofapproximately 4 amps, such as illustrated schematically in FIG. 8A. Thebeam steering coils are properly adjusted to focus the beam pulse intothe V-shaped groove in the collector 37 and biased to one side of thecollimating aperture 38 so that during deflection in the chopper cavityand at only One end of the chopper deflection pattern the particle pulseis directed through the collimating aperture 38 for transmission intothe deflection assembly 14, i.e., only one burst of particles per RFcycle. When the beam pulse is not biased to one side of the collimatingaperture two particle bursts are transmitted per cycle and, for example,when injected into a linear accelerator only one burst will be accepted;the other (displaced 180 in phase) will be automatically rejected by thereversed high electric field in the accelerator. An RF signal such as,for example of S-band frequency is fed into the chopper cavity to sweepthe pulsed particle beam across the collector 37 so that only a portionof the particle pulse is passed through the collimating aperture 38 intothe deflecting assembly each cycle of the oscillating electromagneticfields within the chopper cavity 34, as illustrated schematically inFIG. 8B. The duration of the cycle in the chopping cavity is, for thisexample, about /a nanosecond, and the duration of each individual burstof particles passed from the chopping cavity into the deflectionassembly depends upon the ratio of beam diameter to collimator apertureand the magnitude of scan in the chopper cavity. By passing chargedparticles into the deflecting assembly during only about 10% of thechopper cycle it should be possible to obtain bursts with a duration ofabout nanoseconds. The deflection plates can be used with gas thyratronsor hard tubes. In the latter case higher repetition rates and rise timescan be obtained.

In the deflector, assembly 14 of the deflection plate 52 is grounded anddeflection plate 51 is held at a potential of approximately 10 kv.Similarly, the deflection plate 62 is grounded and deflection plate 61is held at a positive potential of approximately 10 kv. The particledeflection in the region of the second pair of plates 61 and :62 due tothe electric field between the plates is counterbalanced by the magneticfield supplied by the coils 67. The positively charged deflection plates51 and 61 are connected to the thyratrons 55 and 65 which may beindependently triggered to provide pulse length control. During theinitial portion of the pulse, deflection plates 51 and 61 are positivelycharged such that the diverging 'beam pulse passing between the firstpair of plates 51 and 61 is radially deflected from the axis of thedeflection assembly, passes through the field of the focusing lens 46where some image rotation is produced, and is focused in the slot 49 atthe collector 47 radially displaced from the axially located collimatingaperture 48. No deflection action is experienced by the converging beamin passing through the second pair of plates 61 and 62 because of thebiased field condition.

When the first deflector plate 51 is rapidly discharged by triggeringits driver thyratron 55, the beam suddenly becomes symmetrically locatedabout the center line of the deflection system causing the focal pointto sweep along slot 49 in the collector 47 into the collimating aperture48 and provide injection into the prebuncher cavity 71 and subsequentlyinto the accelerating structure. After a suitable controllable timedelay, deflector 61 is discharged by its thyratron 65 and the DCmagnetic bias field remaining in the region between the second pair ofplates causes the beam to be deflected away from the collimatingaperture 48 and swept onto the collector surface 47. One or more choppedand bunched particle bursts may be passed into the acceleratingstructure using this unique deflection assembly.

A number of advantages flow from this structure. The major advantagelies in the capability of continuous and smooth variations of pulselengths from maximum cathode pulse lengths down to fractions of ananosecond. Additionally, not only can very short pulses be produced butalso they can be produced any time during the initial cathode pulse. Forexample, by selecting a portion at the middle of the pulse it ispossible to avoid low current bursts such as occur during the rise andfall times of the pulse. This also permits selection of substantiallyconstant energy particles for insertion into particle accelerators thatcan only accelerate particles over an extremely tight energy spread asrequired in sophisticated nuclear experiments.

The pulsing and accelerating structure in accordance with the presentinvention can produce either a high current or high energy particlepulse for performing a variety of experiments. In this regard with thethree accelerating waveguides 17, 18 and 19 fed with power from the dualoutput klystron in the manner shown in FIG. 1, a high energy and mediumcurrent output can be achieved such as, for example, an output of 25mev. and 270 ma. when each out-put arm of the klystron is carrying 8 mw.By switching the RF power remaining at the output end of the firstaccelerating section 17 from the phase shifter 22 to a load 92 indicatedin phantom in FIG. 1 and directing the second output from the dualoutput klystron via line 91 as shown in phantom in FIG. 1 to the secondaccelerating section 18 instead of the third accelerating section 19, ahigh current medium energy particle output can "be achieved at theoutput end of the second section 18. This high current output can be,for example, on the order of 1 amp at 11 mev. or 2.5 amps at 4 mev. orother values depending upon the operating parameters. This acceleratorconstruction permits the trapping and bunching fields which vary fromcavity to cavity in the initial accelerating waveguide 17 to be utilizedand maintained for both the high current and high energy operationswhile the phase shifter 22 between the first and second acceleratingsections 17 and 18 provides a means of varying the beam energy andcounteracting the small phase shifts due to heavy beam loading.

In order to utilize the high current particle pulse from the output ofthe second waveguide section 18, the third waveguide section 19 must bedecoupled from the particle beam so that the net transfer of energy fromthe beam to the RF structure is zero. One possible arrangement to avoidbeam losses in the third accelerating waveguide section 19 is todisplace the third accelerating section 19 from the axis of theremaining accelerating structure 17 and 18. However this constructionrequires mechanical movement of accelerating guide 19 as well as therequisite opening of the high vacuum system.

As an aspect of the present invention provision is made for changing theoperating temperature of the third accelerating waveguide 19 to phaseslip the microwave circuit of that accelerating Waveguide from theparticle beam transmitted therethrough thereby permitting passage of theparticle beam through accelerating waveguide 19 without energy loss dueto induction of fields in the waveguide. Decoupling of this section isaccomplished by adjusting the heater 20 to change the temperature of thecooling fluid and thereby the operating temperature of the waveguide. Inthis manner the phase relationship between the particle bunches and thetraveling wave continuously varies in such a manner that the nettransfer of energy can be controlled to produce a zero net transfer ofenergy from the particle pulses of the RF structure.

By way of example, by changing the normal operating temperature of anSband frequency waveguide as described in the example above from, forexample, 40 C. to approximately 75 C. the waveguide can be decoupledfrom the high current, medium energy particle pulse emanating fromsection 18 so that this particle pulse can be directed through waveguide19 without substantial loss of beam energy.

While the invention has been described with respect to a three sectionlinear accelerator it can be utilized with accelerators having a greaternumber of sections. Also the heater 20 is utilized to schematicallyillustrate a temperature control means. A cooling unit could beutilized.

While the invention has been described above with reference topositioning of the chopping cavity upstream of the prebunching cavitythis construction requires proper phasing between the signals as appliedto the separate cavities and control of the induced fields in theprebuncher activity due to the chopped beam passing therethrough.Referring now to FIG. 6, there is shown a structure wherein thisbeam-induced field is avoided. As shown, a pulse of charged particles isgenerated in the beam generating assembly 12 and passed through achopping cavity 34 directly into the beam deflection and injectionassembly 14 without clipping of the pulse on a collector. In thedeflection assembly 14 the successive pairs of plates 51-52 and 61-62are utilized to sweep the pulse of particles across the collector 47downstream thereof for passage of the portion of the pulse through thecollimating aperture 48. In this arrangement the positions of the plates61 and 62 are reversed so that instead of moving across the collimatingaperture from one side of the other the pulse is moved into the aperturefrom one side an then moved back out of the aperture on the same side.The prebunching cavity 81 is positioned between the second pair ofplates 61 and 62 and the collector 47. This cavity 81 which ispreferably a circular TM mode cavity is provided with an input disc wall82 having an elongate slot 83 aligned with the deflection path of thecharging particle pulse and an output disc wall 84 having a similarlyoriented slot 85.

In the operation of the device shown in FIG. 6 the chopper deflectedpulse of particles introduced into the cavity 81 is swept across thecollecter 47 by operation of the deflection plates in a manner similarto that described above with reference to FIGS. 1-5 to pass a portion ofthe pulse through the collimating aperture during each cycle of thechopper cavity. After passage of the particles through cavity 81 thepulse is bunched due to the influence of an RF signal coupled thereintoby a coupling member 86. The end result is a beam path which positionsthe beam for passage through the collimating aperture 48 during aportion of the cathode pulse as shown in FIG. 80. Only one collector isutilized and induced fields in the prebunching cavity due to onlyclipped portions of the beam passing therethrough are avoid d.

Naturally many modifications can be made in the construction of thepresent invention without departing therefrom. For example, thecounterbalancing magnetic field produced by the coils 67 can be omittedand both plates 61 and 62 maintained at the same positive potentialuntil the end of the desired pulse length at which time one of theplates is discharged to ground. While the slot 49 in the collector 47 inthe structure of FIGS. 1-5 is designed for sweeping the particle beamforwardly into the collimating aperture 48 and again forwardly out ofthe collimating aperture 48, it is obvious that the potentials on thedeflecting plates can be selected for deflecting the particle beamforward into the collimating aperture 48 and then rearwardly out of thecollimating aperture 48. Additionally, the chopping cavity can be placeddownstream of the prebunching cavity; in this case velocity modualtionof the beam through the chopping cavity will result in an RF deflectionpattern correspondingly modified, and this effect may be used to enhancethe selection of varied electrons for a tighter phase spread ofresultant prebunched beams.

While the chopping cavity having a transverse magnetic field arrangedacross its center path has been described primarily for passage of onlya portion of the particle beam therethrough, an arrangement of a pair ofsuch cavities would also provide an extremely accurate beam positionsignal. For example, when located on the beam center line of a linearaccelerator two rectangular TE cavities connected together and orientedat right angles can be adjusted to indicate in which quadrant the beamcharge center of gravity lies. Similarly such a device with each cavityenergized at the fundamental RF frequency but out of electrical phasecan deflect a beam passing therethrough to produce a circular scan atthe RF cyclic frequency. Such a device can act as a bunch length monitoras a bunched beam of the same frequency made to pass through this devicewill produce only an arc of a circle such that the arc length divided bythe full circumference and multplied by 360 gives the actual bunchlength.

Since many changes can be made in the above construction and manyapparently widely different embodiments of the invention can be madewithout departing from the scope thereof it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An accelerating structure including a plurality of acceleratingwaveguides, means for directing radio frequency energy into each of saidwaveguides and for decoupling said radio frequency energy from one ofsaid waveguides, means for directing charged particles through saidwaveguides for interaction with radio frequency energy therein foracceleration of said particles to relativistic velocities, means forcooling each said waveguides, and means for heating said one waveguideto change the dimensions of said one waveguide and electrically decouplesaid one waveguide from the particles passing therethrough.

2. An accelerating structure for providing high current medium energycharged particle beams and medium current high energy charged particlemeans comprising:

means for creating a beam of charged particles,

a plurality of accelerating waveguide sections including at least first,second and third accelerating sections for interacting with said beam,

input power means connetced to said first section;

a first phase shifting means connected between said input power meansand said second and third sections for selectively directing power tosaid second section for high current medium energy beams and to saidthird section for medium current high energy beams,

a second phase shifting means connected to said first and secondsections,

a load connected to said first section, and said first section includingmeans for selectively directing power from said first section to saidload for providing high current medium energy beams and to said secondphase shifting means for providing medium high energy beams.

3. The accelerating structure of claim 2 including means forelectrically decoupling at least one of said sections from the particlebeam passing therethrough.

4. The accelerating structure of claim 3 wherein said decoupling meansincludes means for selectively changing the temperature of said one ofsaid sections.

5. The accelerating structure of claim 4 wherein said temperaturechanging means comprises a heater.

6. The method of changing the mode of operation of an accelerator from amedium current high energy first mode to a high current medium energysecond mode;

said accelerator comprising in said first mode means for creating a beamof charged particles; first, second and another accelerating waveguidesections for interacting with said beam; means coupling input power tothe waveguide input of said first section and to the waveguide input ofsaid other section; and coupling means between the waveguide output ofsaid first section and the waveguide input of said second section;

said method of changing to said second mode comprising the steps of:

decoupling the waveguide output of said first section from the waveguideinput of said second section;

decoupling said input power from the Waveguide input of said othersection and coupling said input power to the waveguide input of saidsecond section;

and decoupling said other section from said beam.

7. The method of changing the mode of operation of an accelerator from ahigh current medium energy first mode to a medium current high energysecond mode.

said accelerator comprising in said first mode means for creating a beamof charged particles, first and second accelerating waveguide sectionsfor interacting with said beam, a phase shifter, input power meanscoupled to the Waveguide input of said first section and to said phaseshifter, and means coupling said phase shifter to the waveguide input ofsaid second section, said method of changing to said second modecomprising the steps of:

decoupling said phase shifter from said second section,

coupling the waveguide input of a third accelerating waveguide sectionto said phase shifter to interact with the beam from the second section,

and coupling a second phase shifter between the waveguide output of saidfirst section and the waveguide input of said second section.

8. In a charged particle beam device having means for passing a beam ofcharged particles through a waveguide section in a manner which willcause energy loss from the beam to the waveguide section at a giventemperature of the Waveguide section, the method of decoupling saidwaveguide section from said beam by changing the temperature of theWaveguide section to change the dimension of the waveguide section andphase slip said waveguide from said beam to cause substantially zero nettransfer of energy from the beam to the Waveguide section.

References Cited UNITED STATES PATENTS 2,813,996 11/1957 Chodorow3155.42 2,993,143 7/1961 Kelliher et a1 3l539 3,133,227 5/1964 Brown etal 3155.42 3,147,396 9/1964 GoerZ et a1. 3155.42

JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner.

